Metal oxide (MOX) gas sensors are a well-established technology and are based on the deposition of a metal oxide film onto sensing electrodes defined on or within a suitable substrate. The substrate could be a ceramic or, more recently, a silicon substrate. The deposition process could use a thin film technology, such as sputtering, atomic layer deposition or chemical vapour deposition, or a thick film technology such as screen printing, drop coating or ink jetting. In the latter case the film could be deposited in the form of an ink or paste where metal oxide grains are held in suspension in a suitable vehicle, often comprising of organic solvents. This vehicle generally needs to be driven off the powder and any organic compounds decomposed to leave an uncontaminated metal oxide. Furthermore, the metal oxide grains generally need to be “fired” to form a mechanically robust, stable and porous structure which adheres to the substrate and the sensing electrodes.
The drying and organic decomposition processes generally require temperatures up to ˜300° C. but the firing process requires higher temperatures. These temperatures are dependent on many variables such as the starting metal oxide material, the dopants and/or catalysts added to the materials, the metal oxide grain size, the film thickness and the required final structure and porosity. Generally these temperatures exceed 500° C. so care needs to be taken not to adversely affect the quality of the other materials in the sensor such as the substrate, the sensing electrodes and any interconnects.
The use of ceramic substrates generally alleviates some of these sensitivities to processing at elevated temperatures but the use of silicon substrates, particularly CMOS compatible silicon substrates causes some concern. For example, CMOS wirebond pads are generally Aluminum which will readily degrade at temperatures greater than about 400° C. which renders them unsuitable for wire-bonding or providing an Ohmic interconnect. Addition of CMOS circuits to the substrate for control, processing or memory functions are equally at risk to elevated temperatures.
Typical annealing processes using standard conduction and/or convection ovens compromises the final quality of the substrate and any circuits or interconnects contained therein.
Most MOX firing/heating is performed indiscriminately so is not CMOS compatible unless lower than optimum temperatures are deployed which results in compromised sensor quality and performance, as demonstrated in U.S. Pat. No. 8,669,131 B1.
To date various methods are deployed for selective heating such as laser annealing or built in heaters. Both of these methods are slow and costly so not suited to high volume manufacture.
Other methods involve the use of thin films such as CVD or sputtering which do not generally require a post firing process, as demonstrated in US 20110290003 A1.
For metal oxide processing it is possible to use flame spray pyrolysis which utilises a shadow mask to ensure selective deposition and simultaneous firing. This method prevents the user from making changes to the material properties such as porosity and grain size that are essential to produce a quality gas sensor. This method also relies on a shadow mask for selecting the deposition area as the deposition technique is indiscriminate and therefore wasteful of materials that can be very costly as they are likely to contain noble metal dopants and/or catalysts. This is demonstrated in US 20120094030 A1.
Conventional rapid thermal processing (RTP) has been used for firing ceramic films but does not leverage the advantage of selective area heating due to the process and structure used. This is demonstrated in WO 2010129411 A2.